1. Field of the Invention
The present invention relates to a liquid ejection apparatus and a gas processing method, and more particularly, to the structure of an ejection head which ejects liquid droplets from nozzles, and to determination technology and processing technology for gas in an ejection head.
2. Description of the Related Art
An inkjet recording apparatus has been widely used as a recording apparatus which prints and records images that have been captured by a digital still camera, and the like. The inkjet recording apparatus includes a plurality of nozzles in a head, and records a prescribed image on a recording medium by ejecting ink droplets onto the recording medium from the nozzles. An inkjet recording apparatus with a line type head (full line head) corresponding to the breadthways dimension of the recording medium, has been known in which an image is printed on the whole surface of the recording medium while the line type head and a recording medium are moved relatively to each other, in a prescribed movement direction. In the inkjet recording apparatus of this type, a higher printing speed and improved productivity can be achieved in comparison with a serial method in which an image is printed on the whole surface of the recording medium by scanning the recording medium with the head (recording head) in the breadthways direction of the recording medium a plurality of times while causing the recording medium to be moved by a prescribed distance in a direction substantially perpendicular to the scanning direction of the head.
The full line head typically includes a single common liquid chamber for a plurality of nozzles and pressure chambers. If bubbles (which is also referred to as “gas” simply and which includes air bubbles, for example) that are generated in the common liquid chamber are incorporated into the pressure chambers and the nozzles, then ejection abnormalities may occur. Consequently, various technologies have been proposed for removing the gas incorporated in the common liquid chamber (or causing the gas to be dissolved into ink), and thereby preventing gas from infiltrating into the nozzles and the pressure chambers.
Japanese Patent Application Publication No. 11-42795 discloses a composition in which the ink inside the main tank is supplied by means of a main pump to a sub tank, the ink inside the sub tank is supplied to an inkjet head via a main deaeration device, a dissolved oxygen meter, a three-way valve, and the like, and if the value of the amount of dissolved oxygen in the liquid is high, then the three-way valve is switched in such a manner that the liquid in the sub tank is returned via a circulation path connected to the three-way valve, whereby bubbles and dissolved oxygen in the ink, which may cause ink ejection failures and ejection instabilities, are removed from the ink channel without wasting ink, and the bubbles and dissolved oxygen in the ink are thereby prevented from being incorporated into the inkjet head.
Japanese Patent Application Publication No. 2003-182116 discloses a composition in which the pressure value inside the ink supply channel is measured under conditions in which an ink supply channel from a recording head to an ink tube is closed by a valve device, and the pressure is reduced or raised, and a restoration operation is controlled in accordance with the volume of the bubbles accumulated inside the ink supply channel as estimated on the basis of this measurement result.
Japanese Patent Application Publication No. 2002-144604 discloses a composition in which the liquid ejection head includes piezoelectric elements of shear-mode type and a manifold that distributes liquid to the respective pressure chambers, and a voltage is applied between a common electrode formed on the piezoelectric element and an electrode formed in the manifold, and a value that depends on the presence or absence of air bubbles in the liquid is measured in accordance with the conductance caused by application of the voltage, the presence or absence of air bubbles being judged on the basis of this measurement result.
However, in the invention described in Japanese Patent Application Publication No. 11-42795, a dissolved oxygen meter is provided in an ink channel between the inkjet head and the sub tank, and the gas inside the inkjet head is determined indirectly on the basis of the value of this dissolved oxygen meter. In other words, the amount of dissolved gas inside the inkjet head is not measured directly. Since it is difficult to measure the amount of dissolved gas inside the inkjet head accurately, then in the composition disclosed in Japanese Patent Application Publication No. 11-42795, there is a concern that the dissolved gas may turn into bubbles as a result of temperature change inside the inkjet head, or the like, if liquid containing a large amount of dissolved gas is supplied to the inkjet head. Moreover, commonly known dissolved oxygen meters are constituted of consumable items, such as electrodes, separating films, an electrolyte, or the like, and replacement of these consumable items is required.
In the invention described in Japanese Patent Application Publication No. 2003-182116, a pressure measurement device which measures the pressure inside the ink supply channel is provided in the vicinity of an ink tank which is separated from the recording head, and the amount of gas inside the ink supply channel is estimated on the basis of the pressure of the ink supply channel in the vicinity of the ink tank. Therefore, it is difficult to accurately estimate the amount of gas in the ink inside the recording head (this measurement corresponds to determining the sum total of the volume of the bubbles inside the recording head and the ink supply channel). Furthermore, if the amount of gas thus estimated exceeds a threshold value, then a restoration operation of suctioning ink from the ejection ports is carried out in the recording head, and a large amount of ink is consumed when this restoration operation is performed.
The invention described in Japanese Patent Application Publication No. 2002-144604 determines a value which changes depending on the presence or absence of bubbles in the liquid, in accordance with the conductance produced by application of voltage between the common electrode formed on a diaphragm and the electrode formed inside the manifold, and hence there are concerns about decline in the determination accuracy. Cases may arise where a large error occurs in the determination value, depending on the composition of the determination circuit (the accuracy of the determination circuit). Moreover, if a bubble is detected, then a restoration operation is carried out by suctioning the ink via the nozzle holes, and therefore a large amount of ink is consumed when a restoration operation is carried out.